The present invention is directed to devices based on molecular electronics, and preferably to devices based on molecular electronics comprising self-assembled, ordered molecular electronic films.
There has been a recent surge of interest in molecular electronics (see, e.g., (Ball 2000; Reed and Tour 2000)) and organic electronic materials (reviewed in a Nature news feature: (Voss 2000)). This is driven, in part, by the recent demonstration of potentially useful devices. Examples are a programmable logic element (Collier, Wong et al. 1999) and a molecule with remarkable negative differential resistance (Chen, Reed et al. 1999). Despite this progress, fundamental questions remain unanswered.
For example, one (or a few) benzene dithiol molecules pass(es) a current on the order of a microamp at a bias of 4V (Reed, Zhou et al. 1997) whereas the (almost identical) xylyl dithiol molecule passes less than a nanoamp at a similar bias (Datta, Tian et al. 1997). The three orders of magnitude difference is unlikely to be accounted for by the extra carbons in the xylyl dithiol. Which of these experimental results is correct?
Recent work (Cui, Zarate et al. 2001) shows that the problem lies with the highly variable nature of the electrical contact between the molecule and the contacting metal. Most of the applied electric field might be dropped in the gap between the metal contact and one end of the molecule, so that the current-voltage characteristics are severely distorted. Worse still, the electronic properties of the metal-molecule-metal sandwich may become dependent on one of the poorly controlled metal-to-molecule contacts, leading to unreliable and unpredictable behavior. The same work (Cui, Zarate et al. 2001) showed that these problems were removed if the molecule was chemically bonded at each of its ends to the metal connections. This approach has already been employed in the case of benzenedithiol molecules inserted into a so-called xe2x80x98break-junctionxe2x80x99 (Reed, Zhou et al. 1997). In such a device, a molecule, functionalized with a thiol moiety at each end, is assumed to span a small (and otherwise insulating) crack in an electrode. This process relies on accidentally achieving the correct geometry and is extremely hard to control.
Another approach is to make a monolayer of the molecule on an electrode and to contact it by evaporating a metal film onto the top surface (Burghard, Fischer et al. 1996), a process made somewhat more reliable by carrying out the top-metal coating in a micropore (Chen, Reed et al. 1999). Both of these processes rely on physical contact between the molecule and at least one of the metal electrodes.
Yet another approach is to make a pure monolayer of a bifunctionalized molecule and attach a gold particle or electrode to a top thiol moiety while a bottom thiol moiety connects a bottom electrode (Gittins, Bethell et al. 2000). The problem with this approach is that dithiolated molecules may attach to a substrate electrode with both sulfurs, so that each end is tied down and an upper electrode cannot attach covalently to one of the thiol moieties. Even worse, the thiol groups of different molecules can join to form disulfide bridges, so that the molecules join to form polymers of various degrees. Also, it is very difficult to establish that contact is being made to only one molecule.
These experiments have shown that: (i) unambiguous contact to a single molecule is difficult to achieve, (ii) measured currents can be very sensitive to applied stress and (iii) calculated conductivity can disagree with experiment by many orders of magnitud.
Accordingly, a need exists in molecule-based electronics for the formation of reproducible, low resistance electrical contacts between molecules and metal conductors.
The present invention is directed to molecular electronic devices having a self-assembled ordered insulating molecular electronic film having insulating molecules attached at one end to a first electrode, and having conducting device molecules inserted into the insulating molecular electronic film such that the device molecules are chemically attached at the bottom end to a first electrode and the top end to a second electrode.
In one embodiment, the orientation of the molecules in the molecular electronic film is known and controlled.
In another embodiment, the molecular electronic film comprises an ordered alkanethiol self-assembled monolayer composed of alkane chains of approximately the same length as the conducting device molecules that form the molecular electronic device.
In another embodiment of the present invention, the device-forming molecules are inserted into the ordered molecular electronic film by a replacement reaction.
In another embodiment the device-forming molecules are terminated at each end by a thiol moiety. In such an embodiment, one thiol moiety may become attached to the underlying gold substrate during the replacement reaction, while the second thiol moiety is left exposed at the surface of the film after the replacement reaction.
In another embodiment, the surface formed by the molecules is exposed to a fresh solution of nanometer-sized gold particles that eventually attach to the thiols exposed at the surface of the monolayer. In such an embodiment, contact may be made to these chemically attached metal particles, either mechanically or by subsequent evaporation of a top layer.